The invention relates to an electric cigar or cigarette lighter for installation in the dashboard of a motor vehicle such as an automobile, the lighter including an overload protective device.
A cigar lighter of this kind is known from German Offenlegungschrift No. 2,755,620. The apparatus shown therein includes a socket, which can for example, be arranged in the dashboard of a vehicle, and a lighter plug which is inserted into the socket. In the vehicle, the socket of the cigar lighter furthermore serves as an interface between an on-board electrical system of the automobile and accessory devices, e.g. lamps or electrically driven tools, which can be connected to this socket with the aid of accessory plugs.
Cigar lighters of the type shown in the above-mentioned document contain a normal-duty switch-off device, which switches off a current flowing through a glow coil in a glow-coil dish of the lighter plug when a desired temperature of the glow coil is reached.
The cigar lighter known from the above-mentioned prior art reference furthermore contains an overload protective device which, in the case of overheating, which can be caused for example by a failure of the normal-duty switch-off device, brings about an interruption of the electric circuit. The overload protective device also reacts to an excessively high electric current, e.g. a short-circuit current. This is because the overload protective device contains a bimetallic snap-action disc which changes its shape both in the case of heat supply from an external source and in the case of internal heating caused by its own electrical resistance, and thereby interrupts the current circuit.
The overload protective device described above can be arranged in the lighter plug or in the socket. When arranged in the connection to an accessory device, an excess current is detected and switched off.
The overload protective device of the aforementioned known cigar lighter contains a bimetallic disc which has a temperature dependent shape characteristic as illustrated in FIG. 1. Due to a mechanical prestress inherent in the bimetallic disc, the bimetallic disc retains a shape I as the temperature T increases and, after reaching a defined switch-off temperature T.sub.A snaps over into a second stable condition, namely a shape II. After falling below a reset temperature T.sub.R as it cools, the bimetallic disc changes its shape suddenly again, i.e. it reassumes the shape I.
Since the bimetallic snap-action disc is inserted as a switching element into the current circuit, the current circuit is opened in the case of an overload occurring as soon as the switch-off temperature T.sub.A is exceeded and switched on again when the bimetallic snap-action disc has cooled to the reset temperature T.sub.R. An overload is thus not permanently switched off, but renewed heating up occurs at time intervals. In the printed publication mentioned, the overload protective device is designed for this switching behavior in such a way that the temperatures brought about during the switch-on times are not so high that a fire could be caused. Nevertheless, it can be undesired or dangerous to connect the current several times to a current consuming device; at the very least, the battery of the on-board electrical system is discharged in an undesirable manner.